2. Published by
The Institution of Electrical Engineers
Savoy Place
London
WC2R 0BL
November 1993
This edition November 2005
ISBN 0 85296 581 8
The IEE is grateful to the following organisations for permitting their material to be
reproduced:
Nuclear Electric
The cover photograph shows Sizewell B Nuclear Power Station under construction
Nuclear Reactor Types
3. NUCLEAR REACTOR TYPES
Many different reactor systems have been proposed and some of these have
been developed to prototype and commercial scale. Six types of reactor
(Magnox, AGR, PWR, BWR, CANDU and RBMK) have emerged as the designs
used to produce commercial electricity around the world. A further reactor type,
the so-called fast reactor, has been developed to full-scale demonstration stage.
These various reactor types will now be described, together with current
developments and some prototype designs.
Gas Cooled, Graphite Moderated
Of the six main commercial reactor types, two (Magnox and AGR) owe much to
the very earliest reactor designs in that they are graphite moderated and gas
cooled. Magnox reactors (see Fig 1.1(a)) were built in the UK from 1956 to 1971
but have now been superseded. The Magnox reactor is named after the
magnesium alloy used to encase the fuel, which is natural uranium metal. Fuel
elements consisting of fuel rods encased in Magnox cans are loaded into vertical
channels in a core constructed of graphite blocks. Further vertical channels
contain control rods (strong neutron absorbers) which can be inserted or
withdrawn from the core to adjust the rate of the fission process and, therefore,
the heat output. The whole assembly is cooled by blowing carbon dioxide gas
past the fuel cans, which are specially designed to enhance heat transfer. The
hot gas then converts water to steam in a steam generator. Early designs used a
steel pressure vessel, which was surrounded by a thick concrete radiation shield.
In later designs, a dual-purpose concrete pressure vessel and radiation shield
was used.
In order to improve the cost effectiveness of this type of reactor, it was necessary
to go to higher temperatures to achieve higher thermal efficiencies and higher
power densities to reduce capital costs. This entailed increases in cooling gas
pressure and changing from Magnox to stainless steel cladding and from
uranium metal to uranium dioxide fuel. This in turn led to the need for an increase
in the proportion of U235
in the fuel. The resulting design, known as the
Advanced Gas-Cooled Reactor, or AGR (see Fig 1.1(b)), still uses graphite as
the moderator and, as in the later Magnox designs, the steam generators and
gas circulators are placed within a combined concrete pressure-vessel/radiation-
shield.
Nuclear Reactor Types 1
5. Heavy Water Cooled and Moderated
The only design of heavy water moderated reactor in commercial use is the
CANDU, designed in Canada and subsequently exported to several countries. In
the CANDU reactor, (see Fig 1.2) unenriched uranium dioxide is held in
zirconium alloy cans loaded into horizontal zirconium alloy tubes. The fuel is
cooled by pumping heavy water through the tubes (under high pressure to
prevent boiling) and then to a steam generator to raise steam from ordinary water
(also known as natural or light water) in the normal way. The necessary
additional moderation is achieved by immersing the zirconium alloy tubes in an
unpressurised container (called a callandria) containing more heavy water.
Control is effected by inserting or withdrawing cadmium rods from the callandria.
The whole assembly is contained inside the concrete shield and containment
vessel.
Figure 1.2 Schematic: Pressurised Heavy Water Reactor (CANDU)
Water Cooled and Moderated
By moving to greater levels of enrichment of U235
, it is possible to tolerate a
greater level of neutron absorption in the core (that is, absorption by non-fissile,
non-fertile materials) and thus use ordinary water as both a moderator and a
coolant. The two commercial reactor types based on this principle are both
American designs, but are widely used in over 20 countries.
Nuclear Reactor Types 3
The most widely used reactor type in the world is the Pressurised Water
Reactor (PWR) (see Fig 1.3a) which uses enriched (about 3.2% U235
) uranium
6. dioxide as a fuel in zirconium alloy cans. The fuel, which is arranged in arrays of
fuel "pins" and interspersed with the movable control rods, is held in a steel
vessel through which water at high pressure (to suppress boiling) is pumped to
act as both a coolant and a moderator. The high-pressure water is then passed
through a steam generator, which raises steam in the usual way. As in the
CANDU design, the whole assembly is contained inside the concrete shield and
containment vessel.
Figure 1.3a Schematic: Pressurised Water Reactor (PWR)
The second type of water cooled and moderated reactor does away with the
steam generator and, by allowing the water within the reactor circuit to boil, it
raises steam directly for electrical power generation. This, however, leads to
some radioactive contamination of the steam circuit and turbine, which then
requires shielding of these components in addition to that surrounding the
reactor.
Such reactors, known as Boiling Water Reactors (BWRs), (see Fig. 1.3b) are in
use in some ten countries throughout the world.
Nuclear Reactor Types 4
7. Figure 1.3b Schematic: Boiling Water Reactor (BWR)
Water Cooled, Graphite Moderated
At about the same time as the British gas cooled, graphite moderated Magnox
design was being commissioned at Calder Hall in 1956, the Russians were
testing a water cooled, graphite moderated plant at Obninsk. The design, known
as the RBMK Reactor (see Fig 1.4), has been developed and enlarged, and
many reactors of this type have been constructed in the USSR, including the ill-
fated Chernobyl plant. The layout consists of a large graphite core containing
some 1700 vertical channels, each containing enriched uranium dioxide fuel
(1.8% U235). Heat is removed from the fuel by pumping water under pressure up
through the channels where it is allowed to boil, to steam drums, thence driving
electrical turbo-generators. Many of the major components, including pumps and
steam drums, are located within a concrete shield to protect operators against
the radioactivity of the steam.
Nuclear Reactor Types 5
9. A summary of main thermal reactor types
Table 1.1 gives the technical details and the main economic and safety
characteristics of each of the thermal reactor types.
Table 1.1: Summary of the main thermal reactor types
CoolantFuel Moderator
Heat
extraction
Outlet
temp.
Pressure
Spent Fuel
Reprocessing
Steam
Cycle
Efficiency
Main Economic
and Safety
Characteristics
Magnox Natural
uranium metal
(0.7% U
235
)
Magnesium
alloy cladding
Graphite Carbon dioxide
gas heated by
fuel raises steam
in steam
generator
360°C 300 psia Typically
within one
year, for
operational
reasons
31% Safety benefit that coolant
cannot undergo a change
of phase. Also ability to
refuel whilst running gives
potential for high
availability
AGR Uranium
dioxide
enriched to
2.3% U
235
Stainless steel
cladding
Graphite Carbon dioxide
gas heated by
fuel raises steam
in steam
generator
650°C 600 psia Can be stored
under water
for tens of
years, but
storage could
be longer in
dry atmosphere
42% Same operational and
safety advantages as
Magnox but with higher
operating temperatures
and pressures., leading to
reduced capital costs and
higher steam cycle
efficiencies
PWR Uranium
dioxide
enriched to
3.2% U
235
Zirconium
alloy cladding
Light
Water
Pressurised light
water pumped to
steam generator
which raises
steam in a
separate circuit
317°C 2235 psia Can be stored
for long periods
under water
giving flexibility
in waste
management
32% Low construction costs
resulting from design being
amenable to fabrication in
factory-built sub-
assemblies. Wealth
of operating experience
now accumulated world
wide. Off load refuelling
necessary
BWR Uranium
dioxide
enriched to
2.4% U
235
Zirconium
alloy cladding
Light
Water
Pressurised light
water boiling in
the pressure
vessel produces
steam which
directly drives
a turbine
286°C 1050 psia As for PWR 32% Similar construction cost
advantages to PWR
enhanced by design not
requiring a heat
exchanger, but offset by
need for some shielding of
steam circuit and turbine.
Off load refuelling
necessary
CANDU Unenriched
uranium
dioxide (0.7%
U
235
)
Zirconium
alloy cladding
Heavy
water
Heavy water
pumped at
pressure over
the fuel raises
steam via a
steam generator
in a separate
circuit.
305°C 1285 psia As for PWR 30% Good operational record
but requires infrastructure
to provide significant
quantities of heavy water
at reasonable costs.
RBMK Uranium
dioxide
enriched to
1.8% U
235
Graphite Light water
boiled at
pressure, steam
used to drive a
turbine directly
284°C 1000 psia Information not
available
31% Information not available
but operated in
considerable numbers in
the former USSR. Believed
in the West to be inherently
less safe
Nuclear Reactor Types 7
10. Current Developments
Next-Generation (NG) CANDU
NG CANDU is based on the standard proven CANDU design. It introduces new
features:
• Light water reactor coolant system instead of heavy water.
• Use of slightly enriched uranium oxide fuel in bundles rather than natural
uranium fuel.
• Compact reactor core design: core size is reduced by half for same power
output.
• Extended fuel life with reduced volume of irradiated fuel.
• Improved thermal efficiency through higher steam pressure steam
turbines.
The NG CANDU retains the standard CANDU features of on-power fuelling,
simple fuel design and flexible fuel cycles. The steam and turbine generator
systems are similar to those in advanced pressurised water reactor systems.
For safety, NG CANDU design includes two totally independent safety shutdown
systems and an inherent passive emergency fuel cooling capability in which the
moderator absorbs excess heat. The whole of the primary system and the steam
generators are housed in a robust containment to withstand all internal and
external events. (See Figure 1.5)
Figure 1.5 Schematic: NG CANDU Flow Diagram
Nuclear Reactor Types 8
11. British Energy have been involved in a feasibility study of the NG CANDU with
the vendor AECL (Atomic Energy of Canada Limited). This included the feasibility
of the design against UK criteria and in particular licensability of the design.
Advanced Pressurised Water Reactor AP1000
As part of a co-operative programme with the US Department of Energy, the
Westinghouse Company, which is owned by BNFL, have developed an
Advanced PWR with predominantly passive safety systems. Termed the AP600
(600MWe) it is the most up-to-date design licensed in the United States. BNFL
have also developed the AP1000 (1000MWe) with similar safety features to the
smaller version but gaining in economies of scale. The advanced passive design
is a development of the PWR design at Sizewell B.
Key design features of the AP designs are:
• Simplification of standard PWR designs with less piping, fewer valves,
less control cabling and reduced seismic building volumes.
• Modular manufacturing techniques giving a shorter construction schedule
(for the AP600 plant 36 months from first concrete to fuel loading.)
• Passive safety systems using only natural forces such as gravity, natural
circulation and compressed gas. Fans, pumps, diesels and chillers are not
required for safety, nor is operator intervention. A few simple valves are
used to align the passive safety systems when required, in most instances
the valves are 'fail safe' in that on loss of power they move to the safety
position.
• The passive cooling systems include core cooling, providing residual heat
removal, reactor coolant make-up and safety-injection, and containment
cooling which provides the safety related ultimate heat sink for the plant.
• Operating lifetime of 60 years with a design plant availability of 90%+.
• Probabilistic risk assessment has been used as an integral part of the
design process with numerous fine design changes being made as a
result of the PRA studies. The net effect of the overall design approach is
that the predicted core damage frequency is about a factor of 100 better
than current plant designs.
British Energy have been involved in a feasibility study with BNFL/Westinghouse
covering:
• The feasibility of the design against UK criteria and, in particular, the
licensability of the design.
• The technical suitability of AP1000 reactors on existing sites.
• The economic case for the plant and potential funding models.
Nuclear Reactor Types 9
12. Prototype Designs
Designs now being considered for the longer term (ie 2020 - 2030) include:
IRIS - International Reactor, Innovative, Secure
This is based on a small LWR concept with secure safety aspects built in. The
design will be modular and flexible and achieve economic competitiveness.
PBMR - Pebble Bed Modular Reactor
The reactor is a helium-cooled graphite moderated unit of 100MWe which drives
a gas turbine linked to a generator giving up to 50% efficiency. Key design
features:
• Fuel elements are spherical 'pebbles' 60mm in diameter of graphite
containing tiny spheres of uranium dioxide coated with carbon and silicon
carbide. This coating retains the gaseous and volatile fission products
generated in operation.
• The reactor consists of a vertical steel pressure vessel, 6m in diameter
and about 20m high. It is lined with graphite bricks drilled with vertical
holes to house the control rods.
• Helium is used as the coolant and transfers heat to a closed cycle gas
turbine and generator.
• When fully loaded the core contains 310,000 fuel spheres; re-fuelling is
done on-line with irradiated spheres being withdrawn at the base of the
reactor and fresh fuel elements being added at the top.
• The PBMR has inherent passive safety features that require no operator
intervention. Removal of decay heat is achieved by radiation, conduction
and convection. The combination of very low power density of the core
and temperature resistance of the fuel in millions of independent particles
underpins the safety assurance of the design. (See Figure 1.6)
Nuclear Reactor Types 10
13. Figure 1.6 Schematic: Circuit of Pebble Bed Modular Reactor
The PBMR design takes forward the approach originally developed in Germany
(AVR 15MW experimental pebble bed reactor and Thorium High-Temperature
Reactor THTR 300MWe) and is being developed by Eskom, the South African
electrical utility, for application in South Africa initially through a demonstration
plant. Exelon (the major US utility) and BNFL are supporting this venture to
develop and commercialise the PBMR.
Fast Reactors
All of today's commercially successful reactor systems are "thermal" reactors,
using slow or thermal neutrons to maintain the fission chain reaction in the U235
fuel. Even with the enrichment levels used in the fuel for such reactors, however,
by far the largest numbers of atoms present are U238
, which are not fissile.
Consequently, when these atoms absorb an extra neutron, their nuclei do not
split but are converted into another element, Plutonium. Plutonium is fissile and
some of it is consumed in situ, while some remains in the spent fuel together with
unused U235
. These fissile components can be separated from the fission product
wastes and recycled to reduce the consumption of uranium in thermal reactors
by up to 40%, although clearly thermal reactors still require a substantial net feed
of natural uranium.
Nuclear Reactor Types 11
It is possible, however, to design a reactor which overall produces more fissile
material in the form of Plutonium than it consumes. This is the fast reactor in
which the neutrons are unmoderated, hence the term "fast". The physics of this
14. type of reactor dictates a core with a high fissile concentration, typically around
20%, and made of Plutonium. In order to make it breed, the active core is
surrounded by material (largely U238
) left over from the thermal reactor
enrichment process. This material is referred to as fertile, because it converts to
fissile material when irradiated during operation of the reactor.
Due to the absence of a moderator, and the high fissile content of the core, heat
removal requires the use of a high conductivity coolant, such as liquid sodium.
Sodium circulated through the core heats a secondary loop of sodium coolant,
which then heats water in a steam generator to raise steam. Otherwise, design
practice follows established lines, with fuel assemblies clad in cans and arranged
together in the core, interspersed with movable control rods. The core is either
immersed in a pool of coolant, or coolant is pumped through the core and thence
to a heat exchanger. The reactor is largely unpressurised since sodium does not
boil at the temperatures experienced, and is contained within steel and concrete
shields (See Figure 1.7).
The successful development of fast reactors has considerable appeal in
principle. This is because they have the potential to increase the energy available
from a given quantity of uranium by a factor of fifty or more, and can utilise the
existing stocks of depleted uranium, which would otherwise have no value.
Figure 1.7: Sodium-Cooled Fast Reactor
Fast reactors, however, are still currently at the prototype or demonstration
stage. They would be more expensive to build than other types of nuclear power
Nuclear Reactor Types 12
15. station and will therefore become commercial only if uranium or other energy
prices substantially increase.
The British prototype reactor was at Dounreay in Scotland, but has now been
closed on cost grounds. In 1992 the Government announced that all UK research
into fast reactors would cease. The justification for these decisions was the belief
that commercial fast reactors would not be needed in the UK for 30 to 40 years.
Fusion
All the reactors outlined before are fission reactors. Energy can also be produced
by fusing together the nuclei of light elements. This is the process which provides
the energy source in the sun and other stars. The idea of releasing large
amounts of energy by the controlled fusion of the nuclei of atoms such as
deuterium and tritium is very attractive because deuterium occurs naturally in
seawater.
Unfortunately, controlled fusion has turned out to be an extraordinarily difficult
process to achieve. For the reaction to proceed, temperatures in excess of one
hundred million degrees must be obtained and high densities of deuterium and
tritium must be achieved and retained for a sufficient length of time. So far, it has
not proved possible to sustain these requirements simultaneously in a controlled
way. A large number of major projects, including a European collaboration which
has built the Joint European Torus (JET) at Culham in Oxfordshire, have
gradually got closer to reaching the combination of temperature, density and
containment time required for success. Even if this can be achieved eventually,
the process must be capable of being developed in a form which will allow power
to be generated cost effectively and continuously over a long period. It is very
unlikely that this could be achieved until well into the twenty-first century.
Nuclear Reactor Types 13
